This invention relates generally to resistor fabrication and, more specifically, to highly resistive structures formed on a flexible base material and techniques for fabricating such a structure.
A number of commercial applications implement semiconductor devices which may be susceptible to electrostatic discharge. One such device is a semiconductor light emitting diode (LED). LEDs are semiconductor chips that are packaged to emit radiation in response to an applied voltage or current. These LEDs are used in a number of commercial applications such as automotive, display, safety/emergency and directed area lighting. LEDs may be fabricated using any materials which emit visible, ultra-violet or infrared radiation. Thus, LED devices may comprise materials having a p-n junction of semiconductor layers capable of emitting the desired radiation. For example, the LED devices may be fabricated using any desirable III–V semiconductor compound layer, such as GaAs, GaAlAs, GaN, InGaN, GaP, etc., II–VI semiconductor compound layers, such as ZnSe, ZnSSe, CdTe, etc., or IV—IV semiconductor compound layers, such as SiC, for example. Further, the LED devices may also include other layers such as cladding layers, wave guide layers and contact layers.
Currently semiconductor devices, such as LEDs, may be fabricated on a flexible base material such as polyimide to provide flexible device arrays that are conformal to a variety of different shapes for use in a variety of products. For instance, LED arrays may be fabricated on a flexible base material for use in lighting products such as round lamp bulbs, flood lights, cylindrical flashlights, etc. However, semiconductor devices such as GaN based LEDs, for example, are sensitive to electrostatic discharge, because they are fabricated on insulating sapphire substrates. As a result, there is no clear discharge path for electrostatic discharge other than through the LED. Electrostatic discharge through the LED may result in severe damage to the LED by degrading the p-n junction characteristics of the device. It is therefore advantageous to create an alternative path for electrostatic discharge.
One mechanism for minimizing the likelihood of electrostatic damage to the semiconductor devices, such as LEDs, is to incorporate a high value bleed resistor to provide a discharge path for charge dissipation. However, for semiconductor devices fabricated on flexible base materials, the fabrication of resistors may offer certain challenges. It is often difficult to fabricate small high value thin film resistors in the range of 100 kohms–1 Mohm on a flexible base material such as polyimide. Commonly used resistor films such as tantalum nitride (TaN2), nickel chrome (NiCr), and chromium silicide (CrSi) have sheet resistance values in the range of 100–300 ohms/square. At these values, resistors in the range of 100 kohms–1 Mohm are generally patterned in serpentine structures which may cover very large areas. For semiconductor devices fabricated on a flexible base material, the large serpentine structures used to fabricate the highly resistive thin film resistors may be unfeasible. Further, processing on flexible based materials, such as polyimide films, generally require relatively low temperatures (<200° C.) such that the film maintains structural integrity. The lower temperatures required for processing on flexible base materials precludes the use of many of the materials that are typically used to fabricate high value resistors in semiconductor devices, since the materials require processing temperatures greater than 200° C.